The invention described herein may be manufactured and used by or for the Government of the United States of America for Governmental purposes without the payment of any royalties thereon or therefore.
(1) Field of the Invention
The present invention relates to an improved windmill system for generating electrical power which uses a controllable camber blade system.
(2) Description of the Prior Art
Variable camber airfoils have been known for quite some time. As a result, there are numerous patents related to flexible control surfaces for directing fluid flow. For example, U.S. Pat. No. 5,114,104 to Cincotta et al. relates to an articulated control surface which utilizes a moldable control surface that is shaped by contracting and elongating shape memory alloys embedded within the control surface. The shape memory alloys within the control surface contract when heated via an applied electric current and elongated when cooled, i.e. the electric current is removed.
U.S. Pat. No. 5,662,294 to Maclean et al. is another example of a control surface which uses a variable camber design. In this invention, a pliant controllable contour control surface comprises a first flexible facesheet formed to a first initial contour of the control surface, and a second flexible facesheet formed to a second initial contour of the control surface. The first and second facesheets each have a set of prestrained shape memory alloy tendons embedded therein, extending from a leading edge to a trailing edge of the control surface. Each set of the shape memory alloy tendons is separately connected to a controllable source of electrical current such that tendons of the first and second flexible facesheets can be selectively heated in an antagonistic, slack-free relationship, to bring about a desired modification of the configuration of the control surface. A computer based control system is used for maintaining a constant temperature of the antagonists to establish conditions conducive to the stress induced transformation from austenite to martensite, accomplished by causing constant current to flow through the antagonists.
Flexible control surfaces are an advance over the previous art of rigid control surfaces in that they allow control of the fin camber (curvature) as well as angle of attack (pitch). Camber can be used to control flow separation, to increase lift for a given surface area, and to reduce turbulence and noise.
Windmills are alternative energy sources with low environment impact and have been around for many centuries. Numerous attempts have been made over the years to improve the performance of windmills. U.S. Pat. No. 4,003,676 to Sweeney et al.; U.S. Pat. No. 4,160,170 to Harner et al.; U.S. Pat. No. 4,310,284 to Randolph; and U.S. Pat. No. 4,364,708 to David illustrate some of these efforts.
The Sweeney et al. patent relates to a windmill blade that has a rigid leading edge, a rigid root portion, and a movable blade tip. Control of the geometric twist of the windmill blade is accomplished through selection of the axis of rotation for the movable blade tip and by controlling the position of the windmill blade tip with a servo motor or by controlling the position of the windmill blade tip with a spring and a weight. By varying the geometric twist of the blade, the frequency of rotation of the blade is controlled to reduce wind drag upon the blade.
The Harner et al. patent relates to a wind turbine which is connected to an electrical generator to produce electrical power. The pitch angle of the wind turbine blades is controlled in a closed loop manner to maintain either a constant generator speed for isolated power generating stations or when the generator is synchronized to the load, or constant generator output power or shaft torque when the generator is connected to an electrical grid. Open loop acceleration and deceleration schedules are provided to minimize blade stress and shaft torque variations during start up and shutdown transients, limiting blade angle excursions as a function of wind velocity and speed.
The Randolph patent relates to a propeller hub which carries pivotally-mounted blades that are linked to a spring-loaded collar on the propeller shaft for automatic coning and feathering under predetermined high velocity movements along the propeller shaft to change the blade pitch angle during low wind velocity conditions. An airfoil support mounts a propeller shaft and turns therewith to reduce tower shadow effects. This is called a down-wind system meaning the propeller is behind the tower and causes the assembly to rotate into the wind without a tail vane.
The David patent relates to a windmill having blades with both variable pitch and variable spanwise twist. The windmill includes a hub which may be supported on top of a tower for rotation about a substantially horizontal axis. A plurality of blades are provided, each having a root, a tip and a spanwise twistable intermediate section. Linkage mechanisms connect the roots of the blades to the hub to permit selective independent rotation of the roots and the tips of the blades for varying the pitch and the spanwise twist of the blades. A control mechanism is connected to the linkage mechanisms for varying the pitch and spanwise twist of the blades in accordance with a predetermined desired relationship. The aerodynamic properties of the blades are adjusted by the control mechanism to permit the most efficient generation of electric power under most wind conditions, while minimizing the extent of the supporting tower structure necessary to accommodate heavy wind conditions.
As shown in these patents, windmills are configured with two or more blades fastened to a hub. The windmill blades have airfoil cross sections. When wind blows past the blades, it creates lift on the blades, which causes the hub to rotate. The hub spins an electrical generator which produces electrical power. The wind speed at which windmills can practically produce power is limited. At low speeds, the lift on the blades is insufficient to spin the hub and generator. At high speeds, the blades produce high lift that causes windmill spin at a high rate that can cause excessive stresses which damage and potential catastrophically fail the system.
Accordingly, it is an object of the present invention to provide a wind operated power generation system that has improved efficiency over a broad range of wind conditions.
It is a further object of the present invention to provide a wind operated power generation system as above which extends the range of wind speeds at which energy can be practically produced.
The foregoing objects are attained by the wind operated power generation system of the present invention.
In accordance with the present invention, a wind operated power generation system broadly comprises at least two variable camber blades fastened to a rotatable hub. Each of the variable camber blades comprises a flexible material having embedded shape memory alloy members to alter the shape of the blade. The power generation system further comprises a source of electrical power connected to the shape memory alloy members for varying the temperature of each shape memory alloy member and thereby the shape of the blades in response to changes in the speed of the fluid driving the blades. The system still further comprises a power regulator connected to the source of electrical power for regulating the electrical power supplied to the shape memory alloy members and a controller for transmitting a power command signal to the power regulator.
Other details of the wind operated power generation system of the present invention, as well as other objects and advantages attendant thereto, are set forth in the following detailed description and the accompanying drawings in which like reference numerals depict like elements.